Basic components of a computer and other electronic devices include memory, central processing units, a power source and various circuits. Integrated circuit chips are often utilized in such devices. Although the processing speed of the mentioned components can be elevated, the electrical resistance and capacitance of the wires connecting each integrated circuit chip and connecting the integrated circuit chips with the major components are increased due to the miniaturization, which can result in delayed signals and reduced speed.
The ultimate monolithic integration of silicon photonic circuitry and advanced silicon microelectronics may unlock the true power of tomorrow's computers and networks because of vast data capacity, transmission distance, and insensitivity to electromagnetic interference and cross-talk. A key target in the roadmap of silicon photonics is the development of high-performance, current-injected on-chip lasers that can be monolithically fabricated on silicon. While III-V laser junctions have been bonded to silicon waveguides where the emitted light is transmitted, the needed high-quality III-V structures typically cannot be grown directly on the silicon (Si).
It has been highly desirable to have Germanium (Ge) and silicon germanium (SiGe) as potential light emitting media or lasing media. The Ge or SiGe media can be fabricated on silicon utilizing known Group IV epitaxy techniques and with monolithic integration to form silicon electronic integrated circuits. The Γ valley-based direct band gap of unstrained germanium is EgΓ=0.8 eV, which translates to the wavelength of the band edge transition at λ=1.55 μm (the silica optical fiber wavelength).
However, an intrinsic bulk germanium material is an indirect semiconductor with an indirect band gap (EgL=0.67 eV) at L valley, ˜136 meV below the Γ valley minimum. Free electrons tend to populate the L valley first when they are optically or electrically injected into the germanium material. Momentum conservation requires the presence of phonons to assist the radiative electron-hole recombination via the indirect band gap, which significantly lowers the radiative efficiency of germanium. The radiative recombination of Γ valley-electrons and holes through the direct gap will not take place until the L valley is filled by the injected electrons up to the level that energetically matches the minimum of the Γ valley, i.e., ˜136 meV above the bottom of the L valley. The density of the injected currents for the occurrence of radiative recombination and the onset of population inversion in the direct gap (Γ valley) is impractically high in bulk for Ge or SiGe based media.
U.S. Patent Application Publication No. 2007/0105251 to Liu et al. discloses a strain and n-type doping engineering that provides population inversion in the direct bandgap of Ge: the tensile strain decreases the difference between the L valleys and the Γ valley while the extrinsic electrons from n-type doping fill the L valleys to the level of the Γ valley to compensate for the remaining energy difference. The disclosed doping provides a Ge laser structure containing a tensile-strained n+ germanium active layer epitaxially grown over a p-type layer of silicon (Si) or Ge or SiGe, with or without an intermediate region. An optical gain of 400 cm−1 was theoretically predicted for a 0.4% tensile-strained n+ germanium (7.6×1019 cm−3) by considering the free-carrier losses in the heavily doped material.
The very high doping level that is necessary for the L-valley filling in Ge (n≧1019 cm−3) disclosed by Liu et al, will inevitably introduce (1) a large number of nonradiative recombination centers and (2) significantly enhanced the Auger recombination in the material, both of which will compete with and hence reduce the net radiative recombination rate in germanium and provide inefficient light emission. The band tailing effect associated with such heavy doping could also lower the direct bandgap of germanium, turning the emitting wavelength away from the desired 1.55 μm.
U.S. Patent Application Publication No. 2008/0298410 to Cheng et al. discloses infrared emission from a metal-insulator-germanium tunnel diode that occurs at a wavelength near the indirect L-valley gap of Ge. Cheng et al. invoke the idea that holes should be tunnel-injected from a metal gate through an ultra-thin layer of dielectric into the underlying germanium layer where they recombine with electrons to emit light. At a positive gate bias, the electron confinement at the insulator/germanium interface gives rise to the spread of the momentum for the localized electrons, which works together with the phonons, the Ge/oxide interface roughness, and the impurities therein to provide the necessary momentum for radiative recombination. The extracted band gaps from the EL spectra from the Ge metal insulator semiconductor (MIS) tunneling diode are 40 meV lower than the indirect band gap obtained from Varshni's equation at the measurement temperatures, suggesting involvement of the longitudinal acoustic (LA) phonons in the momentum conservation of the radiative recombination. The dominance of the indirect band transition in the Ge MIS tunneling diode is also confirmed by the low carrier density (8×1017 cm−3) in the light emission region, which is not high enough to fill up the L valley for direct band transition. The radiation efficiency of the MIS tunneling diode will essentially be limited by the phonon density state in Ge. In addition, carrier trapping and de-trapping cycles in the insulator often accompany the electron tunneling process, which further reduce the efficiency of the device.
A high-performance, current-injected on-chip laser, light emitting diode or other light emitting device that can be monolithically fabricated on silicon is needed. Such on-chip devices preferably operate at the silica optical fiber communication wavelength (λ=1.55 μm). Preferably, the fabrication of the light emission devices is compatible with Complementary Metal Oxide Semiconductors (CMOS) technology in order to make full use of its billion-dollar industrial tools and facilities.